Fiber-reinforced composites are used in a variety of parts and equipment, including automotive parts, boat parts, building elements, and aircraft parts, among other types of articles. One well established method of making these articles is to place the bare fibers in a mold and then flow in the liquid precursors of a thermoset polymer. Once the precursors have infused through the fibers and filled the mold, a curing stage ensues where the precursor polymerize into a thermoset polymer matrix surrounding the fibers. The fiber-reinforced composite may then be released from the mold and, if necessary, shaped, sanded, or otherwise processed into the final article.
Producing fiber-reinforced composites with widely available glass fibers and uncured thermoset resins is inexpensive and usually does not require complex equipment or extreme processing conditions (e.g., high temperatures) to produce the final articles. Still, there are significant disadvantages associated with making fiber-reinforced thermoset articles, as well as deficiencies with the composites in many applications. One considerable disadvantage with making these articles is the health and safety problems posed by working with uncured thermoset resins. These resins often produce a lot of volatile organic compounds (VOCs), many of which are irritants and even carcinogens. Outgasing VOCs are particularly problematic during curing processes when exothermic polymerization reactions raise the temperature of the composite and increase rate which these compounds evaporate into the surrounding atmosphere. In order to prevent VOC concentrations from exceeding safe limits, expensive ventilation and air treatment equipment is required. This equipment is particularly costly and difficult to maintain for the manufacture of larger composite articles, such as boat hulls and wind-turbine blades.
Another significant problem with making fiber-reinforced thermoset composites is the large amounts of unrecyclable waste they generate. Glass reinforced polyester and epoxy wastes do not easily decompose, making them expensive to landfill. When they are contaminated with toxic precursors, such as epoxy prepregs, they present an even greater environmental challenge. The inability to recycle most fiber-reinforced thermosets also presents a disposal challenge when the articles made from these composites reach the end of their useful lives. The size of this challenge only increases with the size of the articles that must be discarded.
Larger-sized articles present additional challenges for thermoset composites. Thermosets in general cannot be welded or melted, which makes it very difficult, if not impossible, to modify or repair thermoset parts once they have been cured. The high degree of crystallinity that is characteristic of many thermoset polymers also makes the composites prone to fractures that cannot easily be repaired. When fractures and other defects form in larger thermoset articles, often the only option is to replace the article at significant cost.
In view of the significant difficulties with both the manufacture and properties of larger articles made from fiber-reinforced thermoset composites, alternative materials are being investigated. One area receiving interest in replacing or blending the thermoset polymers with thermoplastic polymers. Thermoplastics have advantages over thermosets in many article applications, including a usually superior fracture toughness and chemical resistance that can increase the damage tolerance and useable lifetimes in larger articles. The increased toughness of fiber-reinforced thermoplastic composites often means less material is needed to make an article.
Starting monomers for many thermoplastics are less toxic than those of widely used thermosets, and they produce significantly less noxious gases during article production. Many thermoplastics are also meltable and weldable, which allows larger parts to be repaired instead of prematurely replaced. Thermoplastics are generally also recyclable, which significantly decreases environmental impact and waste disposal costs both during manufacturing as well as at the end of an article's lifecycle.
Unfortunately, thermoplastics also have production challenges including significantly higher flow viscosities than uncured thermoset resins. The flow viscosities of widely used thermoplastic polymers may be orders of magnitude higher than uncured epoxy, polyester, and vinylester thermoset resins, which have flow viscosities comparable to water. The higher flow viscosities makes it difficult for thermoplastic resins to infiltrate a fiber mat and produce a homogeneous polymer matrix composite that is free of voids and seams. Oftentimes, it is necessary to introduce the thermoplastic resin under high temperature or high vacuum, which increases the costs and complexity of manufacturing processes. Thus, there is a need for new methods and materials to make fiber-reinforced thermoplastic composites with reduced production problems and improved bonding between the fibers and the polymer matrix. These and other issues are address in the present application.